In a nutshell, because these "pictures under glass" touchscreen interfaces neglect the tactile aspect of using your hands, they're a poor substitute for the world of objects. Writes Victor:

Pictures Under Glass is an interaction paradigm of permanent numbness. It's a Novocaine drip to the wrist. It denies our hands what they do best. And yet, it's the star player in every Vision Of The Future.

To me, claiming that Pictures Under Glass is the future of interaction is like claiming that black-and-white is the future of photography. It's obviously a transitional technology. And the sooner we transition, the better.